ST. CATHARINES, Ontario - Gay orientation may start in the womb for a boy who was preceded into the world by a band of biological big brothers, a researcher here reported.
ST. CATHARINES, Ontario, June 27 - Gay orientation may start in the womb for a boy who was preceded into the world by a band of biological big brothers, according to a researcher here.
The finding is consistent with previous research showing a correlation between sexual orientation in men and the number of older brothers a man has, according to Anthony Bogaert, Ph.D., of Brock University reported online in the Proceedings of the National Academy of Sciences.
The new finding, in a study of nearly 1,000 men, is that only biological older brothers count. Being raised with adopted brothers or step-brothers with a different mother has no effect on the likelihood of a man growing up gay.
The results "strongly suggest" that the so-called fraternal birth-order effect found in earlier studies has a biological basis, Dr. Bogaert reported, although exactly what that basis is remains unclear.
Dr. Bogaert analyzed four separate cohorts of men, a total of 944. Three of the cohorts were archival and one was specifically recruited to help test the question of whether the fraternal birth-order effect is a result of simply growing up with older brothers or has a basis in biology.
In a linear regression analysis of the whole group, Dr. Bogaert first examined eight sibling variables, including the number of biological brothers men grew up with, both older and younger; the number of non-biological brothers, both older and younger; and similar figures for sisters.
The only significant predictor of homosexuality was the number of biological older brothers. The beta value was 0.10 with a 95% confidence interval from 0.025 to 0.175; a beta value of 0.10 for the number of older brothers indicates a 0.10 standard deviation difference in sexual orientation, holding constant the other predictors in the model.
A second analysis included data on the number of years a man was reared with his brothers and sisters, Dr. Bogaert said. Despite a smaller group for which data was available - only 378 participants could provide such information - the result was the same. Only the number of biological brothers counted, not the time a man spent with them growing up.
"If rearing or social factors associated with older male siblings underlies the fraternal birth-order effect, then the number of non-biological older brothers should predict men's sexual orientation, but they do not," he noted. Instead, having older biological brothers - whether a man grew up with them or not - was the only factor that predicted later homosexuality.
"These results support a prenatal origin to sexual orientation development in men," Dr. Bogaert said, "and indicate that the fraternal birth-order effect is probably the result of a maternal ''memory'' for male gestations or births."
In earlier work, Dr. Bogaert and colleagues estimated that about 28% of homosexual men owe their orientation to fraternal birth-order - "a minority, but not a negligible minority, of all homosexual men."
The results are "compelling," Michigan State University researchers said in an accompanying editorial. They provide evidence that "the social influence of an older brother is irrelevant to whether his younger brother will develop a homosexual orientation."
Instead, wrote Daniel Puts, Ph.D., and colleagues, it's the biological link that the brothers share that affects sexual orientation: "It is the number of older biological brothers the mother carried, not the presence of older brothers while growing up, that makes some boys grow up to be gay," they argued.
One possible explanation, Dr. Puts and colleagues said, is that women may develop an antibody response to the "foreign" tissue of a male fetus. The idea, first proposed in 1985, is that a mother carrying a first son is not exposed to male-specific proteins until delivery, when fetal and maternal blood inevitably mix.
However, an immune response to such proteins will affect subsequent sons, via active transport across the placenta, and might perturb development.
"Whether this is what is really happening for sexual orientation remains to be seen, but it is a provocative hypothesis," the authors wrote.
Noting that Sigmund Freud thought homosexuality was the result of a cold and distant father, they asked: "How much stranger it will be if, instead of the father's psychological rejection, it is the mother's immunological rejection that inadvertently but actively makes her son gay?"